Epoxy resins include a broad class of polymeric materials having a wide range of physical properties. The resins are characterized by epoxide groups, which are cured by reaction with certain catalysts or curing agents to provide cured epoxy resin compositions with various desirable properties. The most common epoxy resins are condensation products of epichlorohydrin and bisphenol A.
It is known to use substituted imidazoles as curing agents for epoxy resins. Co-assigned U.S. Pat. No. 4,931,528, for example, discloses the use of 1-isopropyl-2-methyl imidazole to cure epoxy resins. Japanese Patent No. 63,308,030, to Koei Chem. Ind. KK, discloses an epoxy resin hardener containing 1-alkyl-2-phenyl imidazole. An abstract of the Japanese patent names 1-propyl-2-phenyl imidazole in a list of suitable imidazole curing agents. The imidazole is said to be prepared by dehydrating N-alkyl-ethylene diamine and a nitrile in the presence of a dehydrating catalyst.
Other methods are known for the preparation of imidazoles. In one process, imidazoles are prepared from imidazolines by dehydrogenation of imidazolines over a catalyst comprising nickel in combination with a transition metal, preferably copper and/or chromium. See, for example, co-assigned U.S. Pat. No. 4,927,942, hereby incorporated by reference. Before the discovery of the present invention, it was thought desirable to obtain an essentially quantitative yield of imidazole from a corresponding imidazoline, and to use the essentially imidazoline-free imidazole as the curing agent. Surprisingly, however, Applicants have now discovered that a mixture of 1-isopropyl-2-aryl imidazole and 1-isopropyl-2-aryl imidazoline can be employed as an epoxy resin curative to provide epoxy resins having improved properties. Whereas it was previously thought desirable to dehydrogenate all the imidazoline to imidazole, applicants have discovered, surprisingly, that the presence of a significant amount of imidazoline actually improves the properties of the curative. Additionally, the less strenuous dehydrogenation conditions necessary to obtain the mixture of imidazoline and imidazole make this new curative economically attractive.